1. Field of the Invention
The present invention relates to a control device for an antilock brake system, and more particularly to a control device for an antilock brake system in which braking hydraulic pressure is controlled by a turning angle of a servomotor.
2. Background Art
An antilock brake system (ABS) for performing optimum brake control has been mounted on conventional vehicles. In the ABS, a slip rate is calculated from the rotation speed of a wheel of the running vehicle and the vehicle velocity, and the optimum brake control is performed based on the calculated slip rate.
In an ABS according to the conventional art, as disclosed in Japanese Pre-examination Patent Publication (KOKAI) No.Hei 5-79543 (1993), an actuator for an antilock brake for reducing, maintaining and increasing the braking hydraulic pressure is connected between a master cylinder and a caliper cylinder. The master cylinder is responsible for converting a brake operation to hydraulic pressure. The actuator incorporates a servomotor for displacing a crankshaft of the actuator based on slip rate information of the vehicle, and the crankshaft opens and closes a cut valve through an expander piston, thereby controlling the braking hydraulic pressure applied to the caliper cylinder.
Stopper members are preliminarily provided at an upper limit position and a lower limit position of a turning range of the crankshaft. When the upper limit position or the lower limit position is given as a target angle to the servomotor, the crankshaft or a member in the vicinity of the crankshaft is turned until a positioning member provided in the vicinity of the crankshaft/member comes to collide with the stopper member.
During non-operation of ABS control, the expander piston is retracted to a first side so that the cut valve is maintained in an opened state. The angle of the crankshaft for locating the expander piston at the first side is set at a turning limit for the crankshaft. Therefore, at the end of ABS control, the turning limit of the crankshaft is given as a target angle to the servomotor, and the crankshaft is turned until a positioning member provided at a portion of the crankshaft collides with the stopper member.
According to the conventional arrangements mentioned above, the target angle given to the servomotor is set at an angle exceeding the turning limit of the crankshaft so that the expander piston is retracted to the first end side at the end of ABS control, irrespective of assembly precision of the actuator, crankshaft and the like, and individual variations of component parts.
Therefore, even when the positioning member on the crankshaft side collides with the stopper member and the servomotor is actually locked, the servomotor is continually biased since the actual angle of the crankshaft never reaches the target angle. Accordingly, this results in needless electric current flows to the servomotor.
The present invention overcomes the shortcomings associated with the related art and achieves other advantages not realized by the related art.
It is an aspect of the present invention to provide a control device for an antilock brake system which solves the above-mentioned problems of the conventional art.
It is an aspect of the present invention to provide a control device in which an expander piston displaced by a crankshaft connected to a servomotor can be reliably displaced to a predetermined position corresponding to a turning limit of the crankshaft.
It is a further aspect of the crankshaft to provide improved and reliable expander piston displacement/positioning without consuming needless electric current from the servomotor.
These and other aspects of the present invention are accomplished by a control device for an antilock brake system comprising an input hydraulic chamber in communication with a master cylinder; an output hydraulic chamber in communication with a caliper cylinder of a brake; a cut valve in communication with the input hydraulic chamber and the output hydraulic chamber for performing a cutoff operation; an expander piston controlling a position of the cut valve, the expander piston having a cut valve opening position and a cut valve closing position, the expander piston closing the cut valve by increasing a volume of the output hydraulic chamber in the cut valve closing position; a crank mechanism for displacing the expander piston between the cut valve opening position and the cut valve closing position, the crank mechanism including an actual crank angle; a crank mechanism operating angular range including the actual crank angle, the crank mechanism operating angular range corresponding to any expander piston position between the cut valve opening position and the cut valve opening position, and including the cut valve opening position and the cut valve closing position; a servomotor for controlling the actual crank angle of the crank mechanism according to a target angle, the target angle only being within the crank mechanism operating angular range; and a stopper member for setting a turning limit of the crank mechanism, the turning limit preventing the crank mechanism from turning in excess of a predetermined maximum actual crank angle corresponding to the cut valve opening position, the target angle being set at the predetermined maximum actual crank angle during an ABS non-operational state, and the expander piston being retracted to the cut valve closing position during an ABS operational state.
These and other aspects of the present invention are further accomplished by a method of controlling the abovementioned control device, the method comprising the steps of setting the target angle to a position preceding the predetermined maximum actual crank angle; displacing the expander piston to a position precedent to the cut valve opening position with the servomotor; and maintaining the ABS non-operational state by displacing the expander piston from the position precedent to the cut valve opening position with a spring force from a return spring.
According to the above-described features, the expander piston can be displaced to a position corresponding to the turning limit of the crankshaft or the vicinity thereof without causing the servomotor to consume needless electric current.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.